On 6/28/18 7:59 PM, Ming Lei wrote: > On Thu, Jun 28, 2018 at 09:46:50AM -0600, Jens Axboe wrote: >> Some devices have different queue limits depending on the type of IO. A >> classic case is SATA NCQ, where some commands can queue, but others >> cannot. If we have NCQ commands inflight and encounter a non-queueable >> command, the driver returns busy. Currently we attempt to dispatch more >> from the scheduler, if we were able to queue some commands. But for the >> case where we ended up stopping due to BUSY, we should not attempt to >> retrieve more from the scheduler. If we do, we can get into a situation >> where we attempt to queue a non-queueable command, get BUSY, then >> successfully retrieve more commands from that scheduler and queue those. >> This can repeat forever, starving the non-queuable command indefinitely. >> >> Fix this by NOT attempting to pull more commands from the scheduler, if >> we get a BUSY return. This should also be more optimal in terms of >> letting requests stay in the scheduler for as long as possible, if we >> get a BUSY due to the regular out-of-tags condition. >> >> Signed-off-by: Jens Axboe <[email protected]> >> >> diff --git a/block/blk-mq.c b/block/blk-mq.c >> index b6888ff556cf..d394cdd8d8c6 100644 >> --- a/block/blk-mq.c >> +++ b/block/blk-mq.c >> @@ -1075,6 +1075,9 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx >> **hctx, >> >> #define BLK_MQ_RESOURCE_DELAY 3 /* ms units */ >> >> +/* >> + * Returns true if we did some work AND can potentially do more. >> + */ >> bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head >> *list, >> bool got_budget) >> { >> @@ -1205,8 +1208,17 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, >> struct list_head *list, >> blk_mq_run_hw_queue(hctx, true); >> else if (needs_restart && (ret == BLK_STS_RESOURCE)) >> blk_mq_delay_run_hw_queue(hctx, BLK_MQ_RESOURCE_DELAY); >> + >> + return false; >> } >> >> + /* >> + * If the host/device is unable to accept more work, inform the >> + * caller of that. >> + */ >> + if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE) >> + return false; > > The above change may not be needed since one invariant is that > !list_empty(list) becomes true if either BLK_STS_RESOURCE or > BLK_STS_DEV_RESOURCE > is returned from .queue_rq().
Agree, that's one case, but it's more bullet proof this way. And explicit, I'd rather not break this odd case again. -- Jens Axboe
